1. Field of the Invention
The present invention relates to an electric-power conversion apparatus provided with an electric current detection function capable of accurately calculating the value of the output current of a chopper circuit for supplying electric power to a load.
2. Description of the Related Art
Most of electric-power conversion apparatuses such as inverters or converters each include, as a chopper circuit, an upper stage power semiconductor device connected between a first electric potential and a second electric potential and a lower stage power semiconductor device connected between the second electric potential and a reference electric potential. Based on a desired phase width signal (duty ratio), at least one of the upper stage power semiconductor device and the lower stage power semiconductor device is turned on and off so that electric-power conversion is performed between the first electric potential and the second electric potential. For example, in the case where this kind of electric-power conversion apparatus is applied to an inverter that drives a three-phase AC rotating machine, a DC power source is connected between the first electric potential and the reference electric potential, a desired phase width is appropriately given, and based on the phase width, the upper stage power semiconductor device and the lower stage power semiconductor device are switched, so that the second electric potential with respect to the reference electric potential is converted into an AC voltage. In other words, electric-power conversion is performed in such a way that the first electric potential, which is a DC voltage with respect to the reference electric potential, is converted into the second electric potential, which is an AC voltage. In an inverter that drives a three-phase AC rotating machine, by applying this kind of electric-power conversion to each of the U-phase, the V-phase, and the W-phase, electric-power conversion from a DC power into a three-phase AC power is performed.
In the case where a three-phase AC rotating machine is driven, an electric current supplied by the electric-power conversion apparatus and the torque of the three-phase AC rotating machine are in a close relationship; therefore, insufficient accuracy of detecting the electric current causes a torque ripple, rotational speed ripple, or abnormal noise in the three-phase AC rotating machine.
For example, in the case where there is driven a three-phase AC rotating machine provided in an electric power steering apparatus mounted in a vehicle, a torque ripple emerges as a vibration of the steering wheel; therefore, it is required to accurately detect the electric current so as to reduce the torque ripple as much as possible.
The methods of detecting the output current of an electric-power conversion apparatus include a method utilizing a current transformer and a method utilizing a hole current sensor. Because being not capable of detecting a DC current, the method utilizing a current transformer cannot be adopted in the application in which a three-phase AC rotating machine is in the stop mode or driven at a low rotation speed. In addition, when being mounted in a vehicle, an electric-power conversion apparatus needs to operate at a severe temperature, at a severe humidity, in a vibration, and in a dusty condition; thus, a current detecting resistor is superior to a hole device as the hole current sensor in terms of robustness.
Accordingly, in a conventional electric-power conversion apparatus disclosed in Patent Document 1, there are provided arm circuits for three phases in each of which an upper-arm switching device and a lower-arm switching device are connected in series with each other, the upper-arm switching device is connected with the positive electrode of a DC power source, and the lower-arm switching device is connected with the negative electrode of the DC power source; and current detection resistors that are inserted in series into the respective lower-arm switching devices for at least two phases and each of which detects a current flowing in the lower-arm switching device. By comparing a carrier and a three-phase voltage command wave signal generated based on the foregoing current, the upper-arm and lower-arm switching devices are on/off-controlled through pulse-width modulation, so that a current to be supplied by electric-power conversion apparatus is obtained.
Moreover, for example, a conventional electric-power conversion apparatus disclosed in Patent Document 2 is provided with a current detection means having a shunt resistor provided at the power source side of a load to be PWM-controlled by a drive circuit; in each of the on duration and the off duration of a PWM control signal, the voltage across the shunt resistor is detected; based on the difference between the voltage detected during the on duration and an offset voltage, which is the voltage detected during the off duration, the current flowing in a load is detected.
Furthermore, for example, in a conventional electric-power conversion apparatus disclosed in Patent Document 3, a current current sense resistor is inserted into the output circuit of an inverter that inverts DC electric power into AC electric power for driving a motor, by use of a power device having a gate circuit, and there are provided a sigma-delta modulator that converts the voltage generated across the current sense resistor into a sigma-delta modulated digital signal, an electricity insulating coupler that transfers the digital signal in an electrically insulated manner, a digital filter that demodulates the transferred signal, and a control signal generation circuit that controls the gate circuit of the inverter, based on the output of the digital filter, so that the current to be supplied by the electric-power conversion apparatus is obtained.
Still moreover, for example, in a conventional electric-power conversion apparatus disclosed in Patent Document 4, a motor is driven by an upper-arm switching device and a lower-arm switching device, and there are provided a first reference voltage source that adopts, as a reference, the electric potential of the positive electrode of the battery for the motor, a second reference voltage source that adopts, as a reference, the electric potential of the ground, a shunt resistor provided between a terminal of the motor and the connection point between the upper-arm switching device and the lower-arm switching device, a switching means that outputs the voltage from one of the first and second reference voltage sources, based on the electric potential of the shunt resistor, a voltage dividing means that divides the output of the switching means and the voltage across the shunt resistor so as to generate two voltage signals, and an amplification means that receives the two voltage signals generated by the voltage dividing means and outputs a voltage that increases or decreases at a predetermined amplification factor in accordance with the value and the direction of a current flowing in the shunt resistor, so that the current to be supplied by the electric-power conversion apparatus is obtained.